Very high sensitivity magnetic sensor

ABSTRACT

The present invention relates to a magnetic sensor which comprises a thin deformable membrane ( 3 ) made of a conductive material constituting a first plate of a capacitor and coursed by an electric current, a second capacitor plate consisting of a doped region of a semiconductor substrate ( 1 ), and a layer of a gaseous dielectric ( 6 ) separating the two plates. The membrane deforms due to the effect of the Lorentz force generated by a magnetic field lying in the plane of the membrane and perpendicular to the lines of current. In addition, the invention relates to a process for fabricating this magnetic sensor and to a device for measuring magnetic field.

[0001] The present invention relates to a magnetic sensor comprising a deformable membrane made of a conductive material, to a process for fabricating this magnetic sensor and to a device for measuring the magnetic field.

[0002] Silicon-based magnetic sensors generally use the assymmetry induced by the magnetic field by Hall effect. This is because, after a magnetic field has been applied, an electric field is created perpendicular to the electric current, causing a shift in the lines of current. A potential difference then occurs that can be measured and is proportional to the magnetic field and to the current.

[0003] In the microelectronics field, the trend towards components of increasingly small dimensions means an undesirable reduction in the sensitivity of sensors of this type. To overcome this drawback, it is either necessary to use a low operating temperature or a material having a high electron mobility. The use of a low temperature makes it more difficult to use sensors and increases the complexity and cost of the magnetic-field measurement devices.

[0004] Furthermore, silicon, which is a material of choice in microelectronics, has a relatively low electron mobility (1500 cm².V⁻¹.s⁻¹ at room temperature), which impedes its use in Hall-effect sensors.

[0005] The Applicant has found, surprisingly, that it is possible to remedy the drawbacks of the prior art by producing a magnetic sensor not based on the Hall effect but on the effect of the Lorentz force.

[0006] One subject of the present invention is therefore a magnetic sensor comprising an electrically conductive deformable membrane.

[0007] Another subject of the present invention consists of a process for fabricating this magnetic sensor.

[0008] The present invention also relates to a device for measuring the magnetic field.

[0009] Further characteristics, features and advantages of the invention will become even more apparent on reading the description, on examining the appended drawings and the various examples which follow.

[0010] The subject of the present invention is a magnetic sensor which comprises an electrically conductive deformable membrane, preferably made of single-crystal silicon, constituting a first plate of a capacitor and coursed by an electric current generating lines of current in the membrane, the said membrane deforming under the effect of the Lorentz force generated by a magnetic field lying in the plane of the membrane and perpendicular to the lines of current, a second capacitor plate consisting of a doped region of a semiconductor substrate, and a layer of a gaseous dielectric separating the two plates.

[0011] The membrane has a thickness which varies from 5 to 100 nm, preferably from 10 to 20 nm.

[0012] Furthermore, the semiconductor substrate is preferably made of silicon and the gaseous dielectric is preferably chosen from air and nitrogen, and, most particularly, is air.

[0013] The magnetic sensor of the present invention may be fabricated according to the following process which comprises the steps consisting in:

[0014] providing a substrate made of a semiconductor material, preferably made of silicon, having a main surface;

[0015] depositing a layer of Ge or SiGe alloy on the main surface of the substrate by epitaxy;

[0016] depositing a thin layer of electrically conductive material, preferably silicon, on the layer of Ge or SiGe alloy;

[0017] etching, by means of a mask, two lateral trenches in the thin upper layer of electrically conductive material, the layer of Ge or SiGe alloy and part of the substrate in order to define a central stack consisting of part of the thin layer of electrically conductive material, part of the layer of Ge or SiGe alloy and part of the substrate, and revealing, on two opposed lateral sides of the stack, part of the layer of Ge or SiGe alloy;

[0018] selectively removing, laterally, part of the layer of Ge or SiGe alloy so as to form a cavity bounded by part of the substrate and part of the upper layer of electrically conductive material of the stack, the part of the upper layer of electrically conductive material constituting a thin membrane;

[0019] forming lateral spacers in order to close the cavity beneath the membrane;

[0020] doping the membrane with a dopant having a first type of conductivity, in order to form a first plate of a capacitor;

[0021] doping that part of the substrate which lies beneath the membrane with a dopant of the first type of conductivity, in order to form a second plate of the capacitor; and

[0022] doping the substrate lying at the bottom of the trenches with a dopant of a second type of conductivity, of the opposite type to the first, so as to isolate the second plate of the capacitor.

[0023] The term “SiGe alloy” should be understood in the present invention to mean compounds satisfying the following formulae:

Si_(1-x)Ge_(x),

[0024] with 0<x<1; and

Si_(1-x-y)Ge_(x)C_(y),

[0025] with 0<x≦0.95 and 0<y≦0.05.

[0026] Preferably, an SiGe alloy having a high germanium content, and in particular one having from 10 to 30% by weight of germanium, is used.

[0027] The lateral selective etching of the Ge or of the SiGe alloy may be carried out using one of the techniques well known to those skilled in the art, such as isotropic plasma etching, and etching by means of a chemical oxidizing solution, for example a solution consisting of 40 ml of 50% HNO₃, 20 ml of H₂O₂ and 5 ml of 0.5% HF.

[0028] This process may furthermore include the conventional formation of a silicon oxide layer and the selective etching of this layer for the purpose of producing interconnects. Furthermore, prior to the deposition and to the etching of the SiO₂ layer, an additional step may be carried out which consists in depositing an etching stop layer made of Si₃N₄. This layer, which generally has a thickness of approximately 40 nm, allows the subsequent etching of the structure formed by the process according to the invention to be stopped precisely, without any risk of etching the subjacent layers.

[0029] The present invention also relates to a device for measuring the magnetic field, comprising an oscillating circuit, the capacitor of which consists of at least one magnetic sensor as defined above.

[0030] The electric current coursing the membrane of the sensor may be modulated and correlated with the frequency associated with the magnetic field. This makes it possible to improve the sensitivity to the magnetic field.

[0031] The electric current I flowing in the membrane, due to the effect of the magnetic induction B, generates a force, normal to the surface, which will deform the membrane. By measuring the capacitance C between the membrane and the substrate, this deformation can be measured. This measurement, carried out in differential mode so as to improve the noise immunity, by means of two structures side by side which are coursed by currents flowing in opposite directions, will be carried out either by a synchronous detection method by modulating the current I and by calculating the correlation function between I and C or by direct measurement of the frequency of an oscillator, the frequency of which depends on the value of the capacitance thus produced.

[0032] Furthermore, the magnetic sensor according to the invention may be used with the same principle as a pressure sensor if no current is applied to the membrane.

[0033] The rest of the description refers to the appended drawings in which:

[0034]FIGS. 1a to 1 d show the main steps of the process of the invention for fabricating a sensor according to the invention;

[0035]FIG. 2 shows a device comprising a magnetic sensor according to the invention;

[0036]FIG. 3 shows a device comprising two magnetic sensors according to the invention;

[0037]FIG. 4 shows a device comprising two magnetic sensors according to the invention and a correlator.

[0038] As shown in FIG. 1a, the process starts by forming, on a silicon substrate 1, an SiGe layer 2 and a thin layer of electrically conductive material 3, in the present case silicon, and then a photoresistive resin mask 4.

[0039] The SiGe layer 2 and the silicon layer 3 may be formed on the substrate 1 by epitaxy.

[0040] As shown in FIG. 1b, lateral trenches 5 are etched, using the mask, in the thin Si layer 3, the SiGe layer 2 and part of the substrate 1. Thus, the trenches 5, which define mesa-type stacks, reveal the SiGe layer on two lateral sides of the stack. The SiGe layer is then removed from the stacks in order to form cavities 6, the upper wall of which consists of the remaining part of the thin upper layer 3 of Si which will subsequently form the membrane of the sensor.

[0041] Finally, the resin mask is removed. It is also possible to form, on the internal walls of the cavity, thin layers 7 of dielectric material, for example SiO₂, to serve as a protective barrier.

[0042] As shown in FIG. 1c, spacers 8 made of dielectric material, for example SiO₂, are formed conventionally on each side of the mesa-type stack in order to close off the cavity 6 beneath the membrane 3.

[0043] The process then continues with conventional dopant implantation in the various regions of the structure. Thus, a dopant of a first type of conductivity is implanted in the membrane and the substrate part of the mesa-type stack. The silicon membrane 3 thus constitutes a first plate of a capacitor and the doped part of the substrate of the mesa-type stack, which is a channel region 9 of a semiconductor device, constitutes a second plate of the capacitor.

[0044] The substrate at the bottom of the trenches 5 is then doped with a dopant of a second type of conductivity, of opposite type to the first, in order to form source and drain regions 10 and 11 which electrically isolate the channel region 9 and therefore the second plate of the capacitive structure.

[0045] As shown in FIG. 1d, an Si₃N₄ layer 12 and an SiO₂ layer 13 are formed in succession and then the SiO₂ layer is conventionally etched by means of a mask, the Si₃N₄ layer 12 serving as the etching stop layer, so as to complete the sensor.

[0046] This magnetic sensor can then be used in a device according to the invention. Three examples of devices are illustrated in FIGS. 2 to 4.

[0047] As shown in FIG. 2, an electric current generator is connected to the membrane of the sensor, constituting a first plate of a capacitor, while the channel region of the sensor, constituting the second plate of the capacitor, is connected to an oscillator. The electric current I produced by the generator flows through the membrane, for example as indicated in the figure.

[0048] When this sensor is placed in a magnetic field so that the field is perpendicular to the lines of current and lies in the plane of the membrane, for example as indicated in FIG. 2, the membrane deforms due to the effect of the Lorentz force created and the capacitance of the capacitor varies, also making the frequency of the oscillator vary. The measurement of the frequency of the oscillator is therefore representative, for a constant current I, of the value of the magnetic field applied.

[0049] Moreover, in order to improve the noise immunity, this measurement may be carried out in differential mode by means of two sensors according to the invention (sensor 1 and sensor 2) placed side by side as shown in FIG. 3. These two sensors are mounted so that their membranes are coursed by currents of the same intensity but flowing in opposite directions. Thus, the capacitance variations associated with the magnetic field are added while those associated with the pressure variations are subtracted. Consequently, the frequency of the oscillator depends on the variation in the value of the capacitance due to the magnetic field, independently of the variations in the pressure of the atmosphere surrounding the sensors.

[0050] As shown in FIG. 4, the electric current coursing the membrane of the sensor may be modulated and correlated with the frequency associated with the magnetic field. The current generator is connected to the membrane of the sensor 1 and this membrane is connected to the membrane of the sensor 2 which is earthed. The channel regions of the sensors 1 and 2, constituting the second plates of the capacitors are connected to an oscillator. In this construction, the modulated current I flows in the same direction in each of the membranes of the sensors 1 and 2. The output current of the generator constitutes one of the input signals of a correlator, for example a multiplier followed by a low-pass filter, a second input signal of which is formed by the output signal of the oscillator. The correlator therefore delivers a signal representative of the variations in the magnetic field correlated with the modulated current. In this way, the sensitivity of the magnetic sensor produced is increased. 

1. Magnetic sensor, characterized in that it comprises a thin deformable membrane made of a conductive material constituting a first plate of a capacitor and coursed by an electric current generating lines of current in the membrane, the said membrane deforming under the effect of the Lorentz force generated by a magnetic field lying in the plane of the membrane and perpendicular to the lines of current, a second capacitor plate consisting of a doped region of a semiconductor substrate, and a layer of a gaseous dielectric separating the two plates.
 2. Magnetic sensor according to claim 1, characterized in that the membrane is made of silicon.
 3. Magnetic sensor according to either of claims 1 and 2, characterized in that the membrane has a thickness of 5 to 100 nm.
 4. Magnetic sensor according to claim 3, characterized in that the membrane has a thickness of 10 to 20 nm.
 5. Magnetic sensor according to any one of the preceding claims, characterized in that the substrate is made of silicon.
 6. Magnetic sensor according to any one of the preceding claims, characterized in that the dielectric is a gas.
 7. Process for fabricating a magnetic sensor according to any one of the preceding claims, characterized in that it comprises the steps consisting in: providing a substrate made of a semiconductor material having a main surface; depositing a layer of Ge or SiGe alloy on the main surface of the substrate by epitaxy; depositing a thin layer of electrically conductive material on the layer of Ge or SiGe alloy; etching, by means of a mask, two lateral trenches in the thin layer of electrically conductive material, the layer of Ge or SiGe alloy and part of the substrate in order to define a central stack consisting of part of the thin layer of electrically conductive material, part of the layer of Ge or SiGe alloy and part of the substrate, and revealing, on two opposed lateral sides of the stack, part of the layer of Ge or SiGe alloy; selectively removing, laterally, part of the layer of Ge or SiGe alloy so as to form a cavity bounded by part of the substrate and part of the upper layer of electrically conductive material of the stack, the part of the upper layer of electrically conductive material constituting a thin membrane; forming lateral spacers in order to close the cavity beneath the membrane; doping the membrane with a dopant having a first type of conductivity, in order to form a first plate of a capacitor; doping that part of the substrate which lies beneath the membrane with a dopant of the first type of conductivity, in order to form a second plate of the capacitor; and doping the substrate lying at the bottom of the trenches with a dopant of a second type of conductivity, of the opposite type to the first, so as to isolate the second plate of the capacitor.
 8. Process according to claim 7, characterized in that it furthermore comprises deposition of a layer of silicon oxide and the selective etching of this layer for the purpose of producing interconnects.
 9. Process according to claim 8, characterized in that it comprises, prior to the deposition and to the etching of the SiO₂ layer, an additional step consisting in depositing an etching stop layer made of Si₃N₄.
 10. Process according to claim 9, characterized in that the Si₃N₄ layer has a thickness of approximately 40 nm.
 11. Process according to any one of claims 7 to 10, characterized in that the electrically conductive material of the membrane is silicon.
 12. Process according to any one of claims 7 to 11, characterized in that the membrane has a thickness of 5 to 100 nm.
 13. Process according to claim 12, characterized in that the membrane has a thickness of 10 to 20 nm.
 14. Process according to any one of claims 7 to 13, characterized in that the semiconductor material of the substrate is silicon.
 15. Process according to any one of claims 7 to 14, characterized in that the dielectric is a gas.
 16. Device for measuring the magnetic field, comprising an oscillating circuit, characterized in that the capacitor of the oscillating circuit consists of at least one magnetic sensor according to any one of claims 1 to
 6. 17. Device according to claim 16, characterized in that the electric current coursing the membrane of the sensor is modulated and correlated with the frequency associated with the magnetic field.
 18. Device according to either of claims 16 and 17, characterized in that it comprises two sensors according to any one of claims 1 to 6, the membranes of which are coursed by currents flowing in opposite directions. 